This invention relates generally to fans and blowers for delivering gases at specific flow rates and pressures and more particularly to an impeller or fan wheel of a diagonal-flow fan, the fan wheel being provided with blades each of the shape of a planar surface which affords high performance of the fan substantially equivalent to that of the fan provided with blades each of an ideal shape of a twisted double-curvature surface.
In the fan wheel of an ordinary centrifugal fan of the straight-line, rearwardly inclined (so-called plate-turbo) type, the entrance edges and exit edges of the blades are respectively parallel to the rotational shaft axis. At the same time, when the fan wheel is viewed in its axial direction, each of its blades is linearly formed as it extends toward the periphery of the fan wheel, and each blade has no twist with respect to the axial direction, and cross sections of the blades taken in parallel planes perpendicular to the axis appear to be superposed on each other. Thus, each blade has a planar surface.
Accordingly, the fabrication of these blades is relatively simple. However, the planar shape of the blade of this kind is disadvantageous from the viewpoint of fluid dynamics, and therefore, a fan provided with blades of this type has a low efficiency.
In contrast to the centrifugal fan as described above, a diagonal-flow fan has blades whose entrance edges and exit edges are not parallel to the rotational shaft axis, the radial distance from the shaft axis to each entrance edge varying progressively from one end of the entrance edge to the other, and furthermore, the radial distance from the shaft axis to each exit edge also varying progressively from one end of the exit edge to the other. In addition, each blade must be provided with a complicated double curvature which causes it to have a twist as viewed in the shaft axial direction. These and other features of diagonal-flow fans will be described in detail hereinafter, particularly in comparison with a centrifugal fan.
Theoretically, a diagonal-flow fan should have excellent performance but has not be reduced to practical use because of certain difficulties as will be described hereinafter.